Cyclic ester oligomers (CEOs) have been known for a long time; see for instance U.S. Pat. No. 2,020,298. They are known to be present in varying, usually small, quantities in many linear polyesters and have been isolated from such linear polyesters; see for example A. G. Harrison, “Analysis of cyclic oligomers of poly(ethylene terephthalate) by liquid chromatography/mass spectrometry”, Polymer, 38(10), 2549-2555 (1997) and G. Wick, H. Zeitler, “Cyclic Oligomers in polyesters from diols and aromatic dicarboxylic acids”, Angewandte Makromolekulare Chemie, (1983), 112, 59-94. They are often low viscosity liquids, and it has been known for a long time that they may be polymerized to higher molecular weight linear polyesters by ring opening polymerization; see for instance U.S. Pat. Nos. 5,466,744 and 5,661,214 and references cited therein. This ability to readily form a high molecular weight polymer from a relatively low viscosity liquid has made CEOs attractive as materials for manufacturing processes wherein a low viscosity material is converted to a high molecular polymer in a mold, so that a final shaped part is obtained. They are also attractive candidates as coatings and as encapsulants for electrical components and electronic devices.
However such CEOs have been difficult and expensive to prepare, for example requiring very high dilution conditions and/or using relatively expensive starting materials such as diacyl halides in conjunction with diols and a base to react with the HCI formed; see for instance U.S. Pat. No. 5,466,744. These high manufacturing costs have in many cases prevented the use of CEOs commercially, and therefore lower cost routes to CEOs are of great interest.
Macrocyclic polyester oligomers also can be prepared via the condensation of a dicarboxylic acid chloride with at least one bis(hydroxyalkyl) ester such as bis(4-hydroxybutyl) terephthalate in the presence of a highly unhindered amine or a mixture thereof with at least one other tertiary amine such as triethylamine. The condensation reaction is conducted in a substantially inert organic solvent such as methylene chloride, chlorobenzene, or a mixture thereof. See, for example, U.S. Pat. No. 5,231,161 to Brunelle et al.
These methods suffer from the relatively high cost of dicarboxylic acid chlorides and the need for a base to react with the hydrochloric acid formed in the process. These high manufacturing costs have in many cases prevented the use of macrocyclic ester oligomers commercially, and, therefore, lower cost routes to CEOs are of great interest.
Another method for preparing macrocyclic polyester oligomers or macrocyclic co-oligoesters is the depolymerization of linear polyester polymers in the presence of an organotin or titanate compound. In this method, linear polyesters are converted to macrocyclic polyester oligomers by heating a mixture of linear polyesters, an organic solvent, and a transesterification catalyst such as a tin or titanium compound. The solvents used, such as o-xylene and o-dichlorobenzene are usually substantially free of oxygen and water and solvents must be kept scrupulously dry when titanates are used as catalysts. See, for example, U.S. Pat. No. 5,407,984 to Brunelle et al. and U.S. Pat. No. 5,668,186 to Brunelle et al., and D. J. Brunelle in Cyclic Polymers, Second Edition, J. A. Semlyn (ed.), (2000), Kluwer Academic Publishers (Netherlands), pp. 185-228. The nature of ring-chain equilibrium dictates that the percent yield of cyclic versus linear species drops off significantly as the concentration of starting polymer increases.
More recently, it has been found that polyesters can be made from carboxylic diacids or their diesters and diols using enzymes that catalyze transesterification; see for instance X.Y. Wu, et al., Journal of Industrial Microbiology and Biotechnology, vol. 20, p. 328-332 (1998); E. M. Anderson, et al.; Biocatalysis and Biotransformation, vol. 16, p. 181-204 (1998); and H. G. Park, et al., Biocatalysis, vol. 11, p. 263-271 (1994). In some instances, in such reactions the production of small amounts of CEO coproducts has also been reported; see for instance G. Mezoul, et al., Polymer Bulletin, vol. 36, p. 541-548 (1996). There has also been a study reported on the amounts of CEOs that should be present in such reactions; see C. Berkane, et al., Macromolecules, vol. 30, p. 7729-7734 (1997). The latter study concluded that formation of the CEOs in the enzyme catalyzed reactions followed the same type of rules that govern the formations of these CEOs in nonenzymatic catalyzed reactions, and that only small fractions of CEOs should be produced in such enzymatic reactions unless they were done under very dilute conditions. In all of these references the byproduct alcohol or water from the transesterification/esterification was removed (usually by sparging with an inert gas) to drive the polymeric product to higher molecular weight.
A recent paper, A. Lavalette, et al., Biomacromolecules, vol. 3, p. 225-228 (2002), describes a process whereby an enzymatically catalyzed reaction of dimethyl terephthalate and di(ethylene glycol) or bis(2-hydroxyethyl)thioether leads to essentially complete formation of the dimeric cyclic ester, while use of 1,5-pentanediol leads to a relatively high yield of the dimeric cyclic ester, along with some linear polyester. The formation of high yields of the cyclic with di(ethylene glycol) and bis(2-hydroxyethyl)thioether is attributed to a π-stacking-type short range interaction which favored formation of the dimeric cyclic ester.
U.S. patent application publications 2004-0019177 and 2005-0054809 disclose enzyme-catalyzed processes for the preparation of polyester cyclic ester oligomers. Heretofore, however, it has been unknown in the art how to produce CEOs from the reaction of dicarboxylic acids and/or dicarboxylic acid derivatives with diols or from linear ester oligomers in mixtures of solvents that enhance the formation of the CEOs.
Surprisingly, it has been found that when dicarboxylic acids and/or dicarboxylic acid derivatives with diols or linear ester oligomers are reacted in the presence of an esterification/transesterification enzyme catalyst in an optimal mixture of reaction solvents a significantly increased yield of cyclic ester oligomer can be obtained.